GAS ABSORPTION

Topic Outline

• Introduction

• Basic Principles

• Applications

• Gas – Liquid Equilibrium

• Unit operation for Absorption:

a) Packed tower

b) Plate Column

• Mass Transfer between Phases

ABSORPTION UNIT

• Absorption – between gas and liquid.

• Solutes are absorbed from the gas phase into the

liquid phase.

• Absorption does not destroy the gases.

• It simply transfers the contaminated gas to the

liquid state.

• Stripping or desorption - reverse of absorption

Adsorption vs. Absorption

• Adsorption vs. Absorption– Adsorption is accumulation / adhesion of molecules at

the surface of a solid material (usually activated carbon) in contact with an air or water phase

– Absorption is dissolution of molecules within a phase, e.g., within an organic phase in contact with an air or water phase

Adsorption vs. Absorption

Basic Principles

• The type of contacting liquid chosen depends on the:

1. Solubility of solute (contaminant gases) in the chosen contacting liquid.

- pure water : NH3, acetic acid

2. Chemical reactivity between gas and liquid.

- caustic solution: acid gases, HCl & SO2

- produce a salt

Applications

1. Absorbing SO2 from the flue gases by absorption in alkaline solutions

2. Hydrogenation of edible oils in food industry

- hydrogen gas is bubbled into oil and absorbed.

3. Removal of CO2 from synthesis gases by absorbing it with hot potassium carbonate solution. (in ammonia production)

4. Absorbing dimethyl sulfide from the food processing industry

Applications

Gas-Liquid Equilibrium

• Consider the SO2-air-water system.

• An amount of gaseous SO2, air and water are put in

a closed container and shaken repeatedly at a

given temperature until equilibrium is reached.

• Samples of the gas and liquid are analyzed to

determine the partial pressure pA of SO2 in the gas

and mol fraction xA in the liquid.

Gas-Liquid Equilibrium (con’t)

• The equilibrium plot is shown in Figure 10.2-1.

• The equilibrium relation between pA in the gas phase

and xA can be expressed by a straight line Henry’s

Law equation at low concentration:

pA = H xA

Where H = Henry’s law constant (mol frac gas/ mol

frac liquid)

• The data for some common gases with water are

given in Appendix A.3 (Geankoplis, Transport

Process and Separation Process Principles, 4th ed.,

Prentice Hall)

EQUIPMENT FOR ABSORPTION UNIT

Unit Operation : PACKED TOWER

• A common apparatus used in gas absorption is the

packed tower as shown in Figure 18.1

• The device consist of:

a) cylindrical column or tower

b) gas inlet and distributing space at the bottom

c) liquid inlet and distributor at the top

d) gas & liquid outlets at the top & bottom,

respectively

e) tower packing – supported mass of inert solid

shapes

PACKED TOWER

• The liquid inlet - pure solvent or weak liquor

- is distributed over the top of packing

by the distributor

- uniformly wets the surfaces of the packings

• The distributor - is a set of perforated pipes (Fig. 18.1)

- a spray nozzles in a large towers

• The gas inlet - enter the distributing space below the packing

- flow upward in the packing countercurrent to

the flow of the liquid

PACKINGS

• The packing - provides a large area of contact between

the liquid and gas

- encourage intimates contact between the

phases

• Common dumped packings is shown in Figure 18.2.

PACKINGS• Hollow or irregular packing units – high void spaces

• Intalox saddles – the shape prevents pieces from nesting

closely together

- Increases the bed porosity

• Porosity or void fraction: 60 – 90%

• 3 principal types:

i) dumped packings, (0.25 – 3 inch)

ii) stacked packings, (2 – 8 inch)

iii) structured/ordered packings.

• Made from: plastic, metal or ceramic

Structured Packing

Ceramic Intalox Saddle Packing

Contact between liquid & gas

• Good contact between liquid & gas is the hardest to meet

esp. in large tower

• Channeling – occur at low liquid rates

- some of the packing surface dry

- chief reason for the poor performance

- severe in tower filled with stacked packings

- less severe in dumped packings

- can be minimized by having the ratio of tower

diameter to packing diameter, 8:1

Pressure Drop & Limiting Flow rates

• Figure 18.4 shows typical data for the pressure

drop in a packed tower.

• Pressure drop is due to fluid friction

• Pressure drop - common way of determining if

flooding is occuring / something else goes wrong

inside the absorber.

• The graph is plotted on logarithmic coordinates

for ΔP (inches H20/ft packing) versus the gas

flow rate, Gy (lb/ft2.h)

Loading & Flooding Point• Point K is the loading point

• Point L is the flooding point

for the given liquid flow.

• Loading point is a point

where liquid hold up starts to

increase and caused a

change in the slope of the

pressure drop

• Flooding point is a point

where the gas velocity will

result in the pressure drop

start to become almost

vertical. Liquid rapidly

accumulates, the entire

column filled with liquid.

Material Balances

The overall material balance for a countercurrent absorption process is

Lb+ Vt = Lt+ Vb

where V= vapor flow rate L= liquid flow rate t, b= top and bottom of tower, respectively

The component material balance for

species A is

LbxA,b+ VtyA,t= LtxA,t+ VbyA,b

where yA= mole fraction of A in the vapor phase xA= mole fraction of A in the liquid phase